1. Field of the Invention
The present invention relates to an X-ray mask structure for use in lithography processes in fabrication of large scale integrated circuits (LSI) or micromachines, in which fine patterns are printed on a wafer by means of X-ray exposure.
2. Description of the Related Art
As a result of the very rapid advancement of the technology of very large scale integrated circuits, 4M-DRAMs are now in mass production and 16M-DRAMs or, further, 64M-DRAMs will be in mass production in the near future. As a result of such technology advances, a minimum feature size of 0.5 .mu.m or even 0.25 .mu.m is now required in device fabrication. In device fabrication processes, patterns are transferred from a mask to a semiconductor substrate by use of near ultraviolet light or deep ultraviolet light. However, the minimum feature size has already reached values near the ultimate resolution limit that is attainable by using light of such wavelengths. Furthermore, as the device size becomes smaller, an inevitable reduction occurs in the depth of focus. In view of the above, X-ray lithography technology is generally expected to resolve all the above problems at the same time.
In general, a mask structure for use in an X-ray exposure process comprises fine patterns of X-ray absorber 3 formed on an X-ray transmissive membrane 2 on an adequate supporting frame 1, as shown in FIGS. 1(A) and 1(B).
A known material used for an X-ray absorber includes heavy metal such as gold, tungsten and tantalum. A known material used for an X-ray transmissive membrane for supporting the patterned X-ray absorber includes a silicon compound such as silicon nitride and silicon carbide.
There are a few known methods for forming the pattern of an X-ray absorber, including a plating method in which a resist pattern is used as a stencil, a dry processing method in which a pattern is formed on a thin film of heavy metal by means of a dry process such as reactive ion etching, and a wet processing method in which patterning is performed by using an adequate etching solution.
From the point of view of the effect of Fresnel diffraction which occurs when an X-ray is incident on an X-ray mask, it is reported that an absorber having a cross section with a non-rectangular profile can provide a better pattern transfer to a resist (Japanese Patent Application Laid-Open No. 2-52416).
Furthermore, waviness of a side wall of the absorber avoids the reduction of resolution due to the degradation of contrast of the X-ray intensity at the surface of the resist which results from the reflection of the X-ray incident on the X-ray mask from the side walls of the absorber. This will be discussed in more detail hereinbelow.
In a process for transferring a pattern from an X-ray exposure mask to a resist, the mask (2, 3) is placed at the distance of a few tens of .mu.m from the resist 4 so as to avoid breakage of the mask, as shown in FIG. 2. Then, the resist is exposed to the X-rays 5 through the mask so that the pattern of the absorber 3 is transferred to the resist. However, the exposing X-rays used in this process are not always parallel rays, and the side walls of the absorber are not always vertical depending on the production process. As a result, a portion of the exposing X-rays 5 graze the surface of the side walls of the absorber 3. Thus, a portion of the exposing X-rays 5 are reflected from the side walls of the absorber 3. After reflection, these X-rays propagate in different directions from those of non-reflected X-rays 5, and expose some regions of the resist 4 which are intended to be effectively shielded by the absorber 3 so that the X-rays 5 cannot reach these regions. This results in reduction of resolution of the transferred patterns. This problem can be solved by introduction of waviness on the side walls of the absorber 3.
Reflectivity of an X-ray incident on a substance varies depending on its incident angle. When an X-ray is incident on a substance at an angle almost parallel to the surface, that is, at an incident angle near 90.degree., total reflection occurs and the reflectivity is almost 100%. However, as the incident angle decreases from 90.degree., the reflectivity decreases quickly.
FIG. 3 shows a dependence of reflectivity on incident angle for a 1 nm wavelength X-ray incident on a surface of gold. As seen from this figure, in a range of incident angle from 88.degree. to 90.degree., the reflectivity is larger than 50%. However, as the incident angle decreases, the reflectivity drops quickly, and the reflectivity is less than 1% when the incident angle is less than 85.degree.. When the side walls of the X-ray absorber have a smooth surface with no waviness, the X-rays are incident at angles almost parallel to the surface and a large portion of the incident X-rays are reflected.
On the other hand, as shown in FIG. 4, if the side walls of the absorber 3 have waviness, the X-rays are incident at very small angles on the absorber 3, thus reflectivity becomes very small. That is, if waviness having slopes steeper than 5.degree. with respect to the X-ray direction is introduced on the side walls of the absorber 3, then the reflectivity becomes less than 1%, thus solving the problem of degraded resolution due to the exposure of the resist 4 to the unwanted reflected X-rays.
However, it has been difficult to provide a high precision X-ray mask because of the complicated process required to produce X-ray absorber patterns having arbitrary forms of cross section.